The present invention relates to a variable geometry turbocharger in which rotatable guide vanes surrounding a turbine rotor are arranged within a turbine chamber and a keeping function for keeping a gap between both side walls sectioning the turbine chamber and the guide vanes is provided.
In a turbine of a conventional variable geometry turbocharger, in order to prevent a stick of the guide vanes (that is, the phenomenon that the guide vanes do not move) due to a thermal deformation of both of the side walls sectioning the turbine chamber, it is necessary to provide a gap between both of the side walls and the guide vanes, so that there is a problem that an efficiency of the turbine is reduced when the gap is increased.
In Japanese Patent Examined Publication No. 7-13468, there is shown an embodiment of a variable turbine nozzle provided with rod-like flow passage spacers, in which the flow passage spacers are arranged in an outer edge portion of a whole of the vanes in a radial direction and the axial length of the flow passage spacer is set to be slightly longer than the axial length of the vane. Further, one of the flow passage spacers is preferably press fitted into and fixed within a hole portion provided in a nozzle ring. At least three additional flow passage spacers are arranged so as to be apart from each other with a uniform interval and have a predetermined space between the nozzle ring and an inner wall.
Further, in Japanese Patent Unexamined Publication No. 2-196131, the structure is made such that guide vanes are arranged within a turbine chamber of a turbocharger so as to surround a turbine rotor, at least one of the guide vanes is fixed between both of the side walls of the turbine chamber and a blade angle of the other guide vanes can be adjusted. In accordance with the structure mentioned above, it is possible to keep a gap between both of the side walls of the turbine chamber and the movable guide vane minimum and it is possible to keep a high turbine efficiency.
However, in the case of employing the rod-like flow passage spacer described in the Japanese Patent Examined Publication No. 7-13468, a separation area is formed at the back of each of the flow passage spacers with respect to a stream of gas and a hydraulic energy loss is generated. Further, the separation area extends downstream as a wake so as to reduce an average flow speed at a position of an inlet of the turbine rotor and generate an uneven flow speed distribution in a peripheral direction, whereby a reduction of the turbine efficiency is caused. Further, due to some rotational angles of the guide vane, a stream between the flow passage spacer and the guide vane does not move along the guide vane and there is generated a state that the separation area is formed inside the guide vane. At this time, the wake from the guide vane involving the separation area increases its length downstream, whereby the turbine efficiency is further reduced.
Further, the Japanese Patent Unexamined Publication No. 2-196131 does not refer to the gas stream and does not describe a shape and a size of the guide vane with taking the separation area into consideration.
An object of the present invention is to provide a variable geometry turbocharger which can achieve a high turbine efficiency by preventing a gas stream from being separated at the back of a flow passage spacer and inside a guide vane so as to reduce a hydraulic energy loss and realize a uniform and high average flow speed distribution at a position of an inlet of a turbine rotor.
In accordance with the present invention, a bill-like projection portion is arranged in a part of an outer periphery of a flow passage spacer, and the projection portion is protruded to a turbine rotor side at a predetermined angle or the projection portion is movably provided. Alternatively, a rod-like member is arranged in a part of an outer periphery of a flow passage spacer and the rod-like member is arranged so as to be adjacent to the turbine rotor side at a predetermined angle. Alternatively, a guide vane in which the leading edge side of the rotational shaft is eliminated is arranged in a part of an outer periphery of a flow passage spacer and the rotational shaft is arranged so as to be adjacent to the turbine rotor side at a predetermined angle. Accordingly, the gas stream is prevented from being separated at the back of the flow passage spacer and inside the guide vane, and a high turbine efficiency can be obtained.
Further, a heat resisting cast steel having a little carbon content is employed as a material for a casing sectioning the turbine chamber and a guide vane table. In place of the flow passage spacer, a casing wall and the wall of the guide vane table themselves keep a gap with respect to the guide vane minimum. Accordingly, it is possible to prevent the gas separation from being generated due to the flow passage spacer, and a high turbine efficiency can be obtained.